The main issue I see with increasing party to 4+ is that the game is not being built around it. Personally, I think party of 5 would be the best for me; 4 is playable but pretty limiting in choices, 5 is more flexible while 6 might be a little too many for me to manage.

So if there are 5-6 active party members that will definitely require a rebalance of enemy stats/numbers for a default difficulty level. And with increased enemy numbers maps can become pretty crowded leaving less space to manuever and maybe opening an issue of a party member being more likely to be alpha-striked (probably less relevant as party levels up?). So that might require map rework, and that I don't really see happening. Plus there's multiplayer aspect, and I think getting and commiting 5/6 people to play a long campaign (I'm assuming it will be around DoS2 length, so probably up to 100 hours?) can be harder.